1. Field of the Invention
This invention relates generally to the subject matter of ceramic monoliths, bodies and coating. More particularly, it relates to the peptization of sols of ceramic forming materials, such as aluminum, hafnium, silicon, zirconium, titanium, lanthanum, germanium and tantalum alkoxides, among others, by means of inorganic salts, e.g., nitrates of Al, Zr, Ti, Hf, La, Cr, Ta, Si, Mg, Yt, Co, Ni, Fe, Ge, Rh, Rb and Ba and metal sulfates, sulfides and chlorides of the same elements. This invention also relates to the formation of ceramic material from metal oxides such as, lanthania, thoria, silica, zirconia, alumina, hafnia, titania, chromia, germania, urania, magnesia and tantalum oxide.
2. Description of the Background
It is well known in materials science that predetermined bulk and surface properties may be advantageous for a particular article. These properties may be of a varied nature, including increased toughness as well as high temperature and wear and corrosion resistance. In order to attain this goal, the entire structure of the article may be built utilizing a material which has the desired bulk and surface properties or an article body of desirable properties may be coated with a film having the desired surface properties.
Sol-gel technology involves the preparation of ceramics and glasses by low temperature hydrolysis and peptization of metal oxide precursors (usually organometallic compounds) in solution instead of sintering at high temperatures. By 1975, alumina was synthesized by sol-gel techniques with aluminum sec-butoxide as its organometallic precursor (Yoldas, B.E.(I),"Alumina Sol Preparation from Alkoxides", Amer. Ceram. Soc. Bull. 54(3):289-290(1975); Yoldas, B.E.(II),"A Transparent Porous Alumina", Amer. Ceram. Soc. Bull. 54(3):286-288(1975); Yoldas, B.E.(III),"Alumina Gels That Form Porous Transparent Al.sub.2 O.sub.3 ", J. Mater. Sci. 10:1856-1860(1975). According to Yoldas(I), supra, the process of making alumina involves four basic steps:
(1) hydrolysis of aluminum alkoxides, PA1 (2) peptization of the hydroxide to a sol, PA1 (3) gel formation, and PA1 (4) pyrolysis of the dried gel to alumina.
In the Yoldas(I) process, the hydrolysis of the aluminum alkoxide precursor is conducted with an excess of water in a ratio of 100 moles of water per mole of Al(OC.sub.4 H.sub.9).sub.3, and the peptization is conducted at 90.degree. C. with 0.07 moles of acid per mole of precursor. The peptized sol is then gelled and dried, and the resulting dried samples are calcined at 1200.degree. C., the temperature of formation of alpha alumina. According to Yoldas(II), supra, alpha alumina is the only stable phase above 1150.degree. C.
Sol-gel technology is a chemical synthesis method which results in a particle size limit reduced by at least an order of magnitude to approximately 0.001 micron or less.
Furthermore, sol-gel processing may be conducted with a wide range of ceramic forming materials to form monoliths, bodies and coatings, that are otherwise difficult to achieve by other methods.
In a co-filed co-pending application entitled "Electrodeposition of Ceramic Coatings Using Sol-Gel Processing and Compositions Thereof", U.S. application Ser. No. 841,089, filed Feb. 25, 1986, the present inventors described a novel method of depositing a ceramic coating on a substrate comprising the electrophoretic deposition from organic and inorganic acid-peptized sols.
In yet another co-filed co-pending Application entitled "Deposition of Ceramic Coatings Using Sol-Gel Processing With Application of a Thermal Gradient", U.S. application Ser. No. 841,088, filed Feb. 25, 1986, and now abandoned, the present inventors described a novel method of depositing a ceramic coating on a substrate comprising the thermal deposition from acid-peptized sols.
Metal oxide (really metal hydroxide) sols are presently made by peptization and hydrolysis of the corresponding metal alkoxide, such as, aluminum sec-butoxide (ASB) (in the case of Al.sub.2 O.sub.3) in water with a traditional acid, such as hydrochloric acid, acetic acid, nitric acid and the like. Traditional acids simultaneously break up the precursor, thereby allowing for hydrolysis to occur, and form a repulsive electric double layer around the formed colloids. This mechanism accounts for the fact that colloids (the colloidal particles) are finely dispersed in the sol. However, this same repulsive force inhibits the formation of a dense gel, thus leading to stresses arising when the necessary high drying temperatures are applied thereto. These stresses often cause cracking and fracture during the drying of gel bodies.
Therefore, there still remains a need for an improved method of peptizing sols which leads to more successful making of ceramic monoliths, bodies and coatings for depositing onto a substrate. And also a need for sols having particle sizes in the submicron range which afford better opportunities for making monoliths, bodies and coatings having increased toughness and superior resistance to temperature, wear, corrosion and to cracking and fracture of the monolith, body or coating.